In some cases of current satellite broadcasting, different programs are transmitted with vertically polarized carriers and horizontally polarized carriers through a satellite. To enable a system to selectively receive two orthogonal; polarized signals, a transmission line switch is required for selecting one of the two orthogonal polarized signals from a receiving horn of a microwave antenna.
Accordingly, a low-noise block-down converter (LNB) is provided with the transmission line switch which controls the transmission lines so that only one of the input signals on one of two input lines is amplified and transmitted to an output line and the rest of the input signals are not transmitted to the output line.
FIG. 2 shows an example of a transmission line switch disclosed, for example, in Japanese Laid Open Patent H 2-63210 and is used to fill the above-described requirement. In other words, the transmission line switch disclosed in this reference is adapted so that two orthogonal polarized signals from the receiving horn of the microwave antenna are supplied respectively through input lines 1 and 2 and one of these signals is supplied to a common output line 4 at a connection point 10.
An amplifier la provided on the input line 1 preferably has an amplifier device Q1 formed with a high electron mobility transistor (HEMT) and matching circuits Ml and Ml' which are respectively arranged in the front and rear stages of the amplifier device Ql. Similarly, an amplifier 2a provided on the input line 2 has an amplifier device Q2 formed with a high electron mobility transistor (HEMT) and matching circuits M2 and M2' which are respectively arranged in the front and rear stages of the amplifier device Q2. Matching circuits Ml and Ml' and M2 and M2' are well known in the art and will not be described in further detail.
Bias lines 1b and 2b for respectively supplying a bias to the amplifiers 1a and 2a are connected to the output terminals of the amplifiers 1a and 2a. A DC block circuit 11a for making a DC potential (bias) stay in a transfer-blocked state is arranged between a connection point of the input line 1 with a bias line 1b and a connection point 10 of the input line 1 with the common output line 4. Similarly, a DC block circuit 11b for making a DC potential (bias) stay in a transfer-blocked state is arranged between a connection point of the input line 2 with a bias line 2b and a connection point 10 of the input line 2 with the common output line 4. Each of these DC block circuits 11a and 11b is generally formed with a capacity element or a connection line.
In the configuration shown in FIG. 2, the amplifier 1a or 1b is selectively turned on by the DC bias supplied from the bias line 1b or 2b. The selected amplifier 1a or 1b now in an ON state transfers a signal with a gain exceeding 1.
On the other hand, when the amplifier 1a or 1b is set to OFF, a length L of the line between the amplifier 1a and the connection point 10 and a length L of the line between the amplifier 2a and the connection point 10 are selected so that the impedance as the amplifier in the OFF state is viewed from the connection point 10 is high. Accordingly, one of the signals is transferred to the common output line 4 only through the amplifier remaining ON and the input signal in the amplifier remaining OFF is prevented from being transferred to the output line 4.
However, in the above-described transmission line switch, DC block circuits 11a and 11b should be respectively inserted between bias lines 1b and 2b and the connection point 10 with the common output line 4 to make the bias to be supplied to the amplifier 1a or 2a independent. These DC block circuits 11a and 11b are formed with a capacity element or a connection line and therefore provision of independent DC block circuits 11a and 11b may cause the circuits to be complicated and an unexpected standing wave to occur and this is one of the hindering factors to improvement of the block-down converter.